Carcinogenesis Advance Access originally published online on January 6, 2005
Carcinogenesis 2005 26(4):827-834; doi:10.1093/carcin/bgi012
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Carcinogenesis vol.26 no.4 © Oxford University Press 2005; all rights reserved.
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Dual inhibition of 5-LOX and COX-2 suppresses colon cancer formation promoted by cigarette smoke
1 Department of Pharmacology, Faculty of Medicine, Zhejiang University, Hangzhou, Republic of China, 2 Department of Pharmacology, 3 Department of Physiology, 4 Department of Medicine and 5 Centre of Infection and Immunology, Faculty of Medicine, The University of Hong Kong, Hong Kong, Republic of China
* To whom correspondence should be addressed at: Department of Pharmacology, Faculty of Medicine, The University of Hong Kong, L2-55 Laboratory Block, Faculty of Medicine Building, 21 Sassoon Road, Pokfulam, Hong Kong, Republic of China. Tel: +852 28199252; Fax: +852 28170859; Email: chcho{at}hkusua.hku.hk
Previous studies indicate that the arachidonic acid-metabolizing enzymes COX-2 and 5-LOX are overexpressed during the process of colonic adenoma formation promoted by cigarette smoke. The aims of the present study were to investigate whether there exists a relationship between COX-2 and 5-LOX, and whether dual inhibition of COX-2 and 5-LOX has an anticarcinogenic effect in the colonic tumorigenesis promoted by cigarette smoke. Results showed that pretreating colon cancer cells with cigarette smoke extract (CSE) promoted colon cancer growth in the nude mouse xenograft model. Inhibition of COX-2 or 5-LOX reduced the tumor size. In the group treated with COX-2-inhibitor, the PGE2 level decreased while the LTB4 level increased. In contrast, in the 5-LOX-inhibitor treated group, the LTB4 level was reduced and the PGE2 level was unchanged. However, combined treatment with both COX-2 and 5-LOX inhibitors further inhibited the tumor growth promoted by CSE over treatment with either COX-2-inhibitor or 5-LOX-inhibitor alone. This was accompanied by the downregulation of PGE2 and LTB4. In an in vitro study, we found that the action of CSE on colon cancer cells was mediated by 5-LOX DNA demethylation. In summary, these results indicate that inhibition of COX-2 may lead to a shunt of arachidonic acid metabolism towards the leukotriene pathway during colonic tumorigenesis promoted by CSE. Suppression of 5-LOX did not induce such a shunt and produced a better response. Therefore, 5-LOX inhibitor is more effective than COX-2 inhibitor, and blocker of both COX-2 and 5-LOX may present a superior anticancer profile in cigarette smokers.
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